Voltage indicator



June 30, 1953 SCHNQLL 2,644,134

VOLTAGE INDICATOR Filed July 20, 1949 GENERATOR i POTENI'MMETE/ZZE ENTORSHIELD NATH SCHNOLL hw jm- Patented June 30, 1953 VOLTAGE INDICATORNathan Schnoll, Palisade, N. J., assignor to Industrial Devices, Inc.,Edgewater, N. J., a cor-.

poration Application July 20, 1949, Serial No. 105,703

1 Claim.

The present invention comprises a voltage indicator device which ischaracterized bythe following highly desirable advantages: It isrelatively inexpensive and simple to manufacture, rugged, compact,light-weight, capable of indicating an extremely wide range ofcommercial voltages (both alternating current and direct current) witha'moderate degree of accuracy, and is a highly practical and versatiletool which can be inserted into the vest pocket of a user.

An important aspect of the voltage indicator of the invention is themechanical configuration and engineering design which makes the deviceextremely simple to manufacture and assemble, and enables the device towithstand hard usage, both physical and electrical; without any effecton its ability to perform satisfactorily.

A detailed description of the invention follows, in conjunction with adrawing, wherein:

Fig. 1 illustrates a perspective view of the voltage indicator device ofthe invention, as seen from the front;

Fig. 2 is an exploded view'of the device of Fig. 1 showing the componentparts which go to make up the voltage indicator, in their order ofassemblage;

Fig. 3 is a cross-section of the glow tube and shield taken along theline 3-3 of Fig. 1; and

Fig. 4 shows the circuit diagram of the voltage indicator device of theinvention.

The same parts in the drawing are represented by the same referencenumerals throughout the figures.

An understanding of the electrical principles employed in the voltageindicator device of the invention may be had by first referring to theschematic circuit diagram of Fig. 4, in which a potentiometer 2 is shownwith its terminals connected to insulation covered test leads 4 adaptedto be connected to a source of voltage to be measured. Such a source maybe alternating current or direct current and is represented by the boxlabeled generator. To simplify connections to the generator, the leads 4are provided at the ends with metal prongs 6 extending into fibre orrigid plastic sleeves 8. A glow lamp I9 comprising a pair of parallelelectrodes positioned in a glass envelope has one electrode connected byway of lead I2 to one terminal of the potentiometer through a currentlimiting resistor I4, and its other electrode connected by way of leadIE to a tap IS on the potentiometer. The glow lamp may contain anysuitable inert gas, such as neon or argon, and is adapted to light upwhenever the leads 4 are connected across a source of potentialdifference of suitable value to cause a breakdown of the gas in thetube. A feature of the invention is the U-shaped metallic shield 23which surrounds the rear half of the glow lamp land is connected to leadIt and one terminal'of the lamp, and serves to minimize the effect ofelectrostatic fields caused by the presence of other bodies in thegeneral vicinityof the lamp.

In the operation of the system of Fig. 4, the tap I8 is moved along thepotentiometer 2 until the lamp glows. This indicates that the potentialdifference between leads l6 and I2 exceeds the firing or breakdownvoltage of the glow lamp. As described hereinafter, the tap 18, atthe'start of operation, is at a point of maximum potential differencebetween test leads 4, in whichcase the glow lamp Ii) lights upimmediately upon connecting the leads 4 to the voltage source. Byadjusting the tap l8 to a critical-point at which the lamp Illextinguishers, there is found the voltage which is just below the firingvoltage of the glow lamp, and this is a'function of the voltage of thegenerator. A calibrated scale on the potentiometer enables the voltageof the generator to be read directly from the instrument at a criticalpoint corresponding to the location at which the glow lamp extinguishes.

Fig. 1 shows the meterof the invention ready for use. The potentiometerand glow lamp are encased within a circular housing 22 of. insulat-- ingmaterial, such as Bakelite, having a cover plate 24 also of insulatingmaterial, such as Bakelite. Cover plate 24 is provided witha window 28through which the glow lamp is seen, and with a calibrated scale havingline markings from to 660 representing voltges. The cover plate 24 issecured to spaced shoulders 2| on the housing 22, underneath the plate24, by means of screws 23. A pair of spaced slots 3| at the top of theinsulating housing on opposite sides of one shoulder 2| enables egressof the insulated .covered test leads 4. This shoulder 21 gives strengthto that portion of the housing between the slots 3|, and prevents anytendency of the housing to break under pressure or strain placed on theleads 4. Moreover, the leads 4 extend downward at right angles to themain portion of the leads at thepoints of entry into the housing, thuspreventing movement of that portion of the leads within the housing.This is an important feature of the invention. A rotatable knob 26 issecured to the rotatable metallic shaft 9 of the potentiometer by arecessed screw 21 (note Fig. 2). Shaft 9 of the potentiometer I0 isadapted to pass through a hole I l in the. insulating cover plate. Theknob 26 is made of i insulating material and has. a pointer 29 forindicating the precise marking on the scale at which the glow lampextinguishes. From a practical standpoint, it is preferred'that thecover plate 24 and knob 26 have contrasting colors to enable easyreading of the scale. As an example, the cover plate 22 may be black,the line markings and scale readings 65-600 white, and the knob 26 withits pointer 29 also white. For ease in reading both A.-C. and D.-C.voltages, it is preferred that the insulation covering on the two testleads 4 have different colors, let us say for example, red and black fordifferent leads. V The simplicity of the meter device of the invention,as exemplified by the few components employed and the ease ofassemblage, will be apparent from an inspectionof the exploded'view ofFig. 2. The'glow lamp I6 is mounted above on the periphery of thepotentiometer, and secured thereto solely by virtue of the solderedconnections from its electrodes to the potentiometer. A physically smalllimiting resistor i i is positioned below the glow lamp, as shown. Asuitable washer i3, metallic or insulating, serves to properly space thecover plate 24 from the potentiometer and glow lamp. A threaded locknutwhich engages the threads of a bushing 11 surrounding the shaft 9,firmly holds the potentiometer to the underside of cover plate 2 1.

To obtain a moderate degree of accuracy in the meter of the invention,let us say, a reproducibility of the extinguishing voltage to about :1volt, the glow lamps are carefully selected and aged. I have found thatthere is a residual variation due to changes in the potentialdistribution over the surface of the glass envelope of the lamp and tothe presence of elec trostatic fields due to potential differencesbetween the electrodes of the lamp and other bodies in the generalvicinity of the lamp. Because the potential distribution on the glasslamp envelope and the stray electrostatic fields depend upon the surfaceleakage on the glass and adjacent insulation, the ordinarly glow lampassembly is quitesusceptible to change in ignition and extinguishingvoltages with change in humidity and the position of the hands inholding the device. To overcome this diflicultyand improve the accuracyof the device, I employ a U- shaped brass shield around the center ofthe lamp, and in the rear thereof so as not to obstruct visibility ofthe lamp from the front of the instrument.

The scale shown on the cover plate of Figs. 1 and 2 is adapted to givedirect readings for alternating current voltagesfrom 65 to 660 volts.Thev arrangement is such that the lamp glows im- 4 connected polyphasecircuits by checking the voltage between lines and from lines to ground,to pick out blown fuses by setting the indicator knob just below theline voltage on the scale and testing across the line on the load sideof the fuse. Because the internal resistance of the device is high(approximately megohm) it draws negligible current and can be used tomeasure plate voltages, etc. in high impedance circuits, to checkleakage between the line and metal frames of fiat irons, solderingirons, etc. and provide dependable voltage readings even on voltageoperated circuits such as those using radio-electronic tubes. 7

The rugged construction of the device of the invention enables it tosurvive a large amount of physical abuse. Its construction, shape, andlightness permits it to be carried in the vest pocket of a user orthrown into a kit without injury to the meter.

In the embodiment illustrated in the drawing and successfully tried outin' practice, the following values were used: potentiometer 500,000 ohmsacross the terminals, and limiting resistor 200,000 ohms. The glow lampcontained neon as the inert gas, and was approximately 1" long and A,"wide. The housing and cover plate had a diameter of 1%", and the depthof the hous- 7 shield in such position that said shield surrounds solelythe rear center half .of the glow lamp and said lamp is visible throughsaid window of said cover plate, said potentiometer having a variabletap terminal directly connected to one electrode of saidglowlamp and apair of line terminals coupled across the ends of thepotentiometer, oneof said. line terminals being conmediately upon connecting the testleads d'to a source of voltage. Merely rotating the knob 20 until apoint is reached where the lamp extinguishes will give a voltage readingof the source to be measured. For use with a direct current voltagesupply, the reading should be multiplied by 1.15. In order to determinewhether the circuit voltage is A. C. or D. C., the indicator on knob 26should be set at a point on the scale where the glow lamp glows verybrightly. If the circuit voltage is A. 0., both electrodes of the lampwill glow. If the circuit voltage is D. (3., only one electrode willglow with the red lead connected to the positive side; viz. theelectrode furthest from knob 26 as seen through the window on the coverplate 24 will glow; assuming that the red lead is connected to thelimiting resistor.

The device of the invention is highly versatile and virtually burn-01fproof. It can be used to check whether lines are 110, 220 volts etc.whether A. C. or D. 0., whether delta and wye nected to the otherelectrode of said glow lamp through a current limiting resistor, aconnection fromsaid one electrode to said U-shaped shield, and testleads connected to said terminals and extending out of said housingthrough apertures therein. 7

NATHAN SCI-INOLL.

References Cited in the file of this patent .UNITED STATES PATENTS

